A. Field of the Invention
The present invention relates generally to therapeutic formulations for parenteral administration. In particular, the invention concerns the use of aprotic polar solvents to prepare stable therapeutic formulations by dissolving a therapeutic agent (active ingredient) in an aprotic polar solvent system without the need for drying the peptide from a buffered aqueous solution prior to dissolution in the aprotic polar solvent system. In addition to the active ingredient, a stabilizing excipient(s) may also be included in the formulation, in particular an ionization stabilizing excipient.
B. Description of Related Art
Peptides dissolved in aprotic polar solvents can exhibit enhanced stability and solubility relative to aqueous solutions (see US 2014/0005135 and U.S. Pat. No. 8,697,644); however, direct dissolution of some peptides in an aprotic polar solvent is often not a viable method for preparing a stable and therapeutic composition due to the lack of storage stability. One particular example is glucagon, a 29-amino acid residue peptide hormone used for the treatment of hypoglycemia. Glucagon has an isoelectric point of approximately 7.0, and the molecule is essentially insoluble at neutral pH. Therefore, aqueous solutions must be made either acidic or alkaline before the molecule can be solubilized at therapeutically relevant concentrations. However, acidic and alkaline solutions promote glucagon degradation pathways, and the glucagon molecule has a well-known tendency to fibrillate and form gel-like aggregates in dilute acidic solutions. Therefore, due to the instability of the glucagon molecule, currently available therapeutics are sold as a lyophilized powder that must be reconstituted using a diluent immediately prior to use. By contrast, the glucagon molecule may exhibit enhanced stability and solubility in aprotic polar solvents, such as dimethyl sulfoxide (DMSO).
In addition to peptides and proteins, aprotic polar solvents can also enhance the solubility and stability of therapeutic small molecule drugs relative to aqueous solutions. For example, the small molecule drug diazepam exhibits extremely low solubility in water at neutral pH (<2 mg/mL). To enhance the solubility of diazepam the pH of the aqueous solution is made acidic or alkaline, which in turn increases the rate of hydrolysis and degradation. In contrast, diazepam is very soluble in the aprotic polar solvents dimethyl sulfoxide (DMSO) and n-methyl pyrrolidone (NMP), with a solubility at least an order of magnitude greater in DMSO and NMP relative to neutral water (>50 mg/mL). Additionally, in the absence of formulation excipients, the diazepam molecule is stable in DMSO and NMP, exhibiting stability for at least 6 months in the aprotic polar solvents under accelerated storage conditions (40° C., 75% RH) (see U.S. Pat. No. 9,125,805).
The preparation of non-aqueous peptide formulations via direct dissolution of a peptide in an aprotic polar solvent has been described in the prior art. For example, McMullen (GB Patent Application 2,119,248 A, hereinafter McMullen '248) describes the preparation of insulin solutions by directly dissolving insulin crystals in DMSO. Stevenson et al. (U.S. Pat. No. 5,932,547, hereinafter Stevenson '547), discloses peptide compositions prepared by directly dissolving the peptide in an aprotic polar solvent, such as DMSO or dimethylformamide (DMF). The compositions described by Stevenson '547 are solutions prepared by direct dissolution of the peptide powder as received from a manufacturer or supplier in the non-aqueous solvent, and do not include the use of stabilizing excipients added to the formulation to establish an acceptable ionization profile for preventing physical and/or chemical degradation of the therapeutic molecule. While direct dissolution of a therapeutic molecule in aprotic polar solvents such as DMSO may improve solubility relative to water, the molecule still remains susceptible to multiple physical and chemical degradation pathways. Consequently, direct dissolution in an aprotic polar solvent system has been found to not be suitable pathway for preparing stable formulations of many therapeutic molecules. As an example, at therapeutically relevant concentrations (e.g., 5 mg/mL, or approximately 0.45% (w/w)), solutions prepared by direct dissolution of glucagon powder in DMSO may initially form clear, single-phase compositions, but will eventually form insoluble aggregates within 24 hours at room temperature. Accordingly, direct dissolution of some peptides in an aprotic polar solvent is not a viable method for preparing a stable therapeutic formulation.
The formulations of the present invention are also distinct from those described by Prestrelski et al. (U.S. Pat. No. 8,697,644, hereinafter Prestrelski '644), which discloses peptide formulations prepared by drying the active ingredient (e.g. peptide) from a buffered aqueous solution, and then reconstituting the peptide powder in an aprotic polar solvent. According to this method, the ionization profile that the molecule acquires in the buffered aqueous solution from which it was dried may be retained both in the powder and following dissolution in an aprotic polar solvent system. The ability of a peptide to retain its ionization profile in the dry state from the last aqueous solution from which it was dried is referred to as “pH memory.” However, this approach requires a drying step prior to reconstitution in the aprotic polar solvent, such as freeze-drying or spray-drying, where stabilizing excipients will be required to protect the molecule from the stresses encountered during drying (e.g. thermal stress, mechanical stress, interfacial stress). Further, the addition of a drying step adds significant costs, both in terms of time and expense, to the product development pathway, as the operating parameters and formulation components required for drying the molecule must often be optimized for a particular therapeutic agent, while transfer from the lab-scale to large-scale manufacturing and processing requires further method development and optimization.
Therefore, there remains a need for a formulation platform that couples the stability and solubility provided by aprotic polar solvent systems, but which simplifies and/or expedites the product development pathway by removing the requirement for drying the therapeutic molecule from an aqueous solution prior to reconstitution in a biocompatible aprotic polar solvent system.